The heat-sensitive recording method has the following advantages:
(1) Developing is not required;
(2) When the support is a paper sheet, the quality of the paper may be near that of conventional paper;
(3) Handling of the product is easy;
(4) Coloring density is high;
(5) The recording device is simple and cheap;
(6) There is such an advantage that no noise is generated in the time of recording; and the like.
Due to such advantages, in recent years the heat-sensitive recording method has been used widely in the fields of facsimile and printers.
Under such a background as described above, there is a need for developing a heat-sensitive recording material of improved transparency, which can be directly used for recording with a thermal head, including multi-color process application, or to be used in an over-head projector (abridged as OHP).
However, the conventional transparent heat-sensitive recording material is a so-called transparent heat-sensitive film, which is brought into close contact with a manuscript and then irradiated with light to let the image part of the manuscript absorb infra-red rays, whereby the temperature of the image part is raised, causing corresponding parts of the heat-sensitive recording film to be colored. Therefore, it does not have such heat sensitivity for direct recording with a thermal head as used in a facsimile or the like.
Also, the type of heat-sensitive layer of the heat-sensitive recording material adapted for thermal recording with a thermal head becomes devitrified, so the desired transparency could not be obtained.
The present inventors have developed a novel transparent heat-sensitive material which overcomes the defects of conventional heat-sensitive recording materials. The basic heat-sensitive system used is a combination of a colorless or pale colored base dye precursor (color former) and a color developer as the color-forming system. This type of system is described in JP-A-63-265682 (the term "JP-A" as used herein refers to an "unexamined published Japanese patent application"). According to that document, a transparent heat-sensitive recording material can be obtained by a process comprising the steps of introducing the base dye precursor into microcapsules, dissolving the color developer into an organic solvent difficultly soluble or insoluble in water, emulsifying and dispersing in a water phase the color developer dissolution product, mixing the base dye precursor-containing microcapsules with the dispersion and then coating the mixture on a support.
However, it was found that the transparency of the above-described heat-sensitive recording material depends markedly on the emulsion stability of the color developer of the heat-sensitive layer. For example, when p-hydroxy benzoic acid benzyl ester, which is known as a color developer of high sensitivity in a heat-sensitive recording material, was used, it was liable to crystallize out and to be fogged, thereby adversely affecting transparency.